This invention relates to a vent type injection molding machine of the type wherein volatile substances and moisture which are contained in a molten resin and air presenting in a metal mold cavity are removed before an injection operation, and a method of operating the vent type injection molding machine.
When the resin contains volatile substances and moisture at the time of injection or when air is present in the mold cavity, gases of these substances would be sealed between the surface of the injected resin and the inner surface of the mould cavity so that such defects as silver streaks and resin burning would be resulted. To prevent such defects, the resin may be predried before it is loaded in the injection machine. However, since this method increases the cost, the vent type injection molding machine has been proposed.
One example of the prior art vent type injection molding machine will now be described with reference to FIG. 3, of the accompanying drawings, which comprises a screw 12 contained in a heating cylinder 11 to be reciprocatable and rotatable. A vent port 13 is formed at an intermediate point of the heating cylinder 11 for discharging volatile components and moisture into surrounding air or for sucking them with a vacuum pump, not shown. Unless the resin pressure at the vent port 13 is made to be sufficiently low, the resin tends to flow out and clog the vent port 13, thus disenabling the injection molding machine. Accordingly, as shown in FIG. 3, the screw 12 was divided into two sections, that is a first stage section 12A and a second stage section 12B. With this construction, it has been necessary to design such that the transfer capability of the second stage section 12B will be sufficiently larger than that of the first stage section 12A.
For this reason, it was necessary to increase the entire length of screw 12 which not only increases the cost but also imposes a limit on the screw design. Due to a screw construction necessary to maintain the vent port at a low pressure, the resin tends to stay at the vent portion, thus causing burning of the resin and making it difficult to readily change the resin.
Moreover, with the prior art vent system, as it is difficult to remove the air in the mold cavity, it has been obliged to use an evacuated metal mold. In this case, an evacuation pipe having a sufficiently large cross-sectional area is connected to the metal mold so that it is necessary to use some measure which prevents leakage of the resin through the evacuation pipe at the time of injecting the resin. Moreover, the metal mold must be carefully designed so as not to impair the appearance of the molded product.
FIG. 4 shows one example of a prior art injection molding apparatus utilizing an evacuated metal mold, in which an injection nozzle 21 of the heating cylinder 11 is urged against a stationary metal mold 22. A movable mold 23 is moved toward and away from the stationary metal mold to define a mold cavity 24 therebetween. A cover plate 25 is secured to metal mold 23 or 22, and an evacuating passage 26 is provided through the movable metal mold 23. In order to evacuate at a high speed, the passage 26 should have a large cross-sectional area and open at the peripheral surface of the movable metal mold 23. After the cavity 24 has been surrounded by cover 25, the cavity 24 is evacuated through passage 26. However, this construction requires a large evacuating apparatus having a capacity of evacuating a volume much larger than the actual volume of the cavity 24. Moreover, as it takes a definite evacuation time before mold closure, the time of the molding cycle becomes long.